220 



ELECTRICAL CURRENTS OF THE EARTH. 



(*) jSTeedlo oscillates between 5° and 0°. 



Up to tills moment tlie needle oscillated slightly ; bnt afterwards, 

 under tlie action of the storm, the needle made great oscillations, though 

 always to the side in which the current maintains it, and at the close of 

 the day the deviation seemed lixed at about 00°. It has already been 

 said, that on July 2, with a clear sky and the air warm and dry, the 

 deviation was re-established and fixed at between 15° and UP. 



During a storm on the 7th I kept the electroscope in exercise for a 

 length of time at the intermediate station, and constantly realized great 

 oscillations of the needle, even to zero, when the instrument gave signs 

 of negative electricity or very weak signs of positive electricity. Under 

 strong winds also these oscillations were verified. Again, the usual 

 deviation of the ascending- current increased slowly or rapidly, according 

 as the electroscope indicated a corresponding augmentation in the signs 

 of positive electricity, or a sudden augmentation of the same electricity 

 at the moment of a flash of lightning In many other series of experi- 

 ments, which I deem it needless to report, I have always found in calm 

 and clear days a deviation nearly constant, a result which I have never 

 witnessed during storms nor even on clear days during high winds and 

 great oscillations of the atmospheric pressure. 



I proceetl to describe the experiments executed upon a mixed circuit 

 in which the metallic conductor, an iron wire of 3 millimetres, (^ inch,) 

 and well insulated, had a length of about 45 kilometres, (28 miles.) The 

 two extreme stations, which were Pontedera and Volterra, were at a dif- 

 ference of elevation amounting to 540 metres, (1,772 feet.) The experi- 

 ments were performed in the nights of the 11th and 23d of July, under 

 a calm and serene sky, commencing at 7.30 p. m. and terminating at 

 4.45 a. m. 



In the line between Pontedera and Volterra the telegraphic offices are 

 closed at night; and in order to be more certain that no current of the 

 telegraph could be introduced during the experiments on the line, I pro- 

 vided for an interruption of the latter at each extremity, at the distance 

 of a pole or two from the office; at the point of interruption I soldered a 

 piece of copper Avire covered with gutta-percha, which at the station of 

 Volterra descended immediately into the earth, where it was united to 



